Clamshell Heat Exchangers

ABSTRACT

A clamshell heat exchanger for use in a combustion furnace of an HVAC system is presented that includes in one instance two passageways coupled by a turnaround passageway. The first passageway that receives the combustion products diverges. A cross section of the first passageway resembles a tear drop or air foil with the widest portion closest to the second passageway. The second passageway also diverges from the turnaround portion towards the outlet. The second passageway may include a baffle that forms two flow streams. Other embodiments are presented.

TECHNICAL FIELD

This application is directed, in general, to combustion furnaces for HVAC systems, and more specifically, to clamshell heat exchangers.

BACKGROUND

In gas furnaces used as an aspect of heating ventilating air conditioning (HVAC) systems, a number of heat exchangers are utilized. The heat exchangers may be used in a heating area of the furnace to heat an airstream that passes through the heating area of the furnace. One type of heat exchanger that is used is a “clamshell” heat exchanger. A “clamshell” heat exchanger typically includes two concave halves that are joined either by welding or folding to form a passageway or opening with an inlet and an outlet. To form such a clamshell heat exchanger, individual panel halves, typically formed by stamping mirror images of the combustion chambers into corresponding metal sheets, are coupled together to form the passageway. Clamshell heat exchangers have been used extensively, and yet still, improvements are desired.

The preceding discussion of the background is intended to facilitate an understanding of the present disclosure only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge at the priority date of the application.

SUMMARY

According to an illustrative embodiment, a clamshell heat exchanger for use in a direct combustion furnace of an HVAC system includes a first clamshell half and a second clamshell half. When the first clamshell half is coupled to the second clamshell half, it forms an assembled clamshell heat exchanger. The clamshell heat exchanger further includes an inlet and an outlet, and a first passageway, which is a combustion passageway, having a first end and a second end. The inlet is proximate the first end. The clamshell heat exchanger also includes a turnaround passageway having a first end and second end, wherein the second end of the first passageway is fluidly coupled to the first end of the turnaround passageway, and includes a second passageway, which is an exhaust passageway, having a first end and a second end, wherein the first end is fluidly coupled to the second end of the turnaround passageway and wherein the outlet is proximate the second end of the second passageway. The first passageway is fluidly coupled to the turnaround passageway and the second passageway is fluidly coupled to the turnaround passageway to form a flow pathway for combustion products. A lateral cross section of the first passageway has more area above a lateral centerline than below the lateral centerline. In one illustrative embodiment, the lateral cross section of the first passageway is shaped like an upside down tear drop or like an airfoil with a leading edge facing upward.

According to another illustrative embodiment, a clamshell heat exchanger for use in a combustion furnace includes a first clamshell half and a second clamshell half. When coupled, the first clamshell half and the second clamshell half form the clamshell heat exchanger. The clamshell heat exchanger further includes a first passageway coupled to an inlet for receiving combustion products from a burner assembly. The first passageway has a longitudinal length in a first direction and forms a first portion of a flow pathway. The clamshell heat exchanger also includes a turnaround passageway coupled to the first passageway and forming a second portion of a flow pathway that receives fluids going in a first direction and turns the fluids at least 120 degrees. The clamshell heat exchanger further includes a second passageway coupled to the turnround passageway and having a longitudinal length in the first direction and forming a third flow pathway that is opposite in direction to the first flow pathway and that comprises two flow streams. The first passageway has a lateral cross section resembling an airfoil with its leading edge closest to the second passageway.

According to still another illustrative embodiment, a clamshell heat exchanger for a combustion furnace includes a heat exchanger body formed with an inlet and an outlet; a first passageway, turnaround passageway, and a second passageway formed in the heat exchanger body that together define a flow pathway from the inlet through the first passageway, then the turnaround passageway, then the second passageway, and finally to the outlet. An outer wall portion and an inner wall portion of the first passageway diverge from one another. The first passageway is not symmetric about a lateral centerline. In one embodiment, a lateral cross section of the first passageway is shaped like a teardrop having a widest portion closest to the second passageway. Other embodiments and teachings are disclosed herein.

DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the present invention are described in detail below with reference to the attached drawing figures, which are incorporated by reference herein and wherein:

FIG. 1 is an exploded, schematic, perspective view of a portion of a combustion furnace showing one illustrative deployment of an illustrative embodiment of a clamshell heat exchanger according to the disclosure;

FIG. 2 is a schematic, elevation view of an illustrative embodiment of a clamshell heat exchanger for use in a combustion furnace;

FIG. 3A is a schematic, perspective view of the clamshell heat exchanger of FIG. 2 with a cross section cut made at line 3A-3A;

FIG. 3B is a detail of the cross section cut of the first passageway of the clamshell heat exchanger of FIGS. 2 and 3A;

FIG. 4 is a schematic, elevation view of an illustrative embodiment of a clamshell heat exchanger for use in a combustion furnace;

FIG. 5 is a schematic, elevation view of an illustrative embodiment of a clamshell heat exchanger for use in a combustion furnace;

FIG. 6 is a schematic, cross section of the clamshell heat exchanger of FIG. 5 taken along line 6-6;

FIG. 7 is a schematic, cross section of the clamshell heat exchanger of FIG. 5 taken along line 7-7;

FIG. 8 is a schematic, cross section of the clamshell heat exchanger of FIG. 5 taken along line 8-8;

FIG. 9 is a schematic, cross section of the clamshell heat exchanger of FIG. 5 taken along line 9-9;

FIG. 10 is a schematic, cross section of the clamshell heat exchanger of FIG. 5 taken along line 10-10;

FIG. 11 is a schematic, cross section of the clamshell heat exchanger of FIG. 5 taken along line 11-11;

FIG. 12 is a schematic, cross section of the clamshell heat exchanger of FIG. 5 taken along line 12-12;

FIG. 13 is a schematic, cross section of the clamshell heat exchanger of FIG. 5 taken along line 13-13;

FIG. 14 is a schematic, cross section of the clamshell heat exchanger of FIG. 5 taken along line 14-14;

FIG. 15 is a schematic, cross section of the clamshell heat exchanger of FIG. 5 taken along line 15-15;

FIG. 16 is a schematic, elevation view of an illustrative embodiment of a clamshell heat exchanger for use in a combustion furnace showing determination of a slope of the first passageway;

FIG. 17 is a schematic, elevation view of an illustrative embodiment of a clamshell heat exchanger for use in a combustion furnace showing determination of a slope of the second passageway;

FIG. 18 is a schematic, cross section of an illustrative embodiment of a clamshell heat exchanger for use in a combustion furnace taken along a portion of the first passageway such as at line 3A-3A of FIG. 2;

FIG. 19 is a schematic, cross section of an illustrative embodiment of a clamshell heat exchanger for use in a combustion furnace taken along a portion of the first passageway such as at line 3A-3A of FIG. 2; and

FIG. 20 is a schematic, elevation view of an illustrative embodiment of a clamshell heat exchanger for use in a combustion furnace.

DETAILED DESCRIPTION

In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is understood that other embodiments may be utilized and that logical structural, mechanical, electrical, and chemical changes may be made without departing from the spirit or scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the invention, the description may omit certain information known to those skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the claims.

In gas furnaces used as an aspect of heating ventilating air conditioning (HVAC) systems, a number of heat exchangers are utilized, such as a “clamshell” heat exchanger. A “clamshell” heat exchanger typically includes two concave halves that are coupled either by welding or folding to form a passageway or opening with an inlet and an outlet. To form such a clamshell heat exchanger, individual panel halves, typically formed by stamping mirror images of the combustion chambers into corresponding metal sheets, are coupled together to form the passageway. The metal sheet may be made with various materials and thickness. In one illustrative embodiment, the clamshell halves are made from 0.74 mm (29 mil) T1-40 EDDS aluminized steel. In other embodiments, the material may be 0.74 mm (29 mil) T1-25 EDDS aluminized steel or DDS aluminized steel, or 409 stainless steel.

With these types of furnaces using clamshell exchangers, air passes through the heating zone from a blower or fan and is heated. The furnaces of this type experience high operating temperatures, which can cause failure within the heating chamber. In this context, the illustrative embodiments of the clamshell heat exchangers, systems, and methods herein may lead to a reduction in thermo-mechanical stress of the clamshell heat exchangers while maintaining combustion, heat transfer, and flue and air-side pressure drops as desired. Moreover, the heat exchangers, systems, and methods may provide improved reliability compared to existing systems and, in some embodiments, allow for lower-cost materials to be used.

One type of furnace that is used in HVAC systems at times is referred to as an “80% gas furnace.” Such a furnace may be one in which approximately 80% of the energy put into the furnace is converted into heat for the purposes of heating a targeted space. Such furnaces may have high operating temperatures, e.g., >1000° Fahrenheit. Other furnaces may be used as well that involve higher efficiencies. Two commercial examples are the LENNOX ML 180 furnace and the LENNOX EL296V gas furnace. The illustrative embodiments herein may be well suited for such furnaces. Unless otherwise indicated, as used throughout this document, “or” does not require mutual exclusivity.

Referring now to the figures and initially to FIG. 1, an illustrative embodiment of a gas furnace 100, which is an “80% gas furnace,” that includes a plurality of clamshell heat exchangers 104. Those skilled in the art will understand that the clamshell heat exchangers described herein may be used with many different types and sizes of furnaces. While the term “clamshell” is used herein, it should be understood that the same may be formed as integral piece as well.

The illustrative gas furnace 100 includes a housing 108 having an opening 112 that in an assembled position is covered by covering 116. A mounting shelf 120, which is formed with opening 124, is positioned within the housing 108 when assembled. A heating zone 128 is formed in a heat exchanger assembly 130, which includes the plurality of clamshell heat exchangers 104.

The clamshell heat exchangers 104 have a configuration to provide a number of potential benefits over previously known clamshell heat exchangers. The plurality of clamshell heat exchangers 104 are coupled to an inlet panel 132, or vest panel. A plurality of combustion inlets 136 on the inlet panel 132 provide combustion products into inlets of each of the plurality of clamshell heat exchangers 104 and a plurality of combustion outlets 140 on a upper portion of the inlet panel 132 receive the combustion products after they have traveled through the plurality of clamshell heat exchangers 104. While a specific number of clamshell heat exchangers 104 are shown in the figure, those skilled in the art will understand that other embodiments may include fewer or more of the clamshell heat exchangers 104. The flow pathway through each clamshell heat exchanger 104 is generally serpentine with a 180 turn as will be described herein. The inlets 136 and outlets 140 may be coplanar on the inlet panel 132, or vest panel.

The furnace 100 further includes a burner assembly 144. The burner assembly 144 may include a thermostatically-controlled solenoid valve 148 or other valve and one more gas inlets (not explicitly shown). Gas orifices (not explicitly shown) are coupled to a manifold 152. Burners within the burner assembly 144 may correspond in a one-to-one fashion with the plurality of inlets 136 although other illustrative embodiments may have more or fewer burners. When assembled, the burner assembly 144 lines up with the heat exchanger assembly 130 such that burners of the burner assembly 144 align with the plurality of inlets 136 to provide combustion products thereto.

The illustrative furnace 100 also includes a draft-inducer assembly 160. The draft-inducer assembly 160 includes a draft-inducing fan 162 fluidly coupled to a manifold 164, which fluidly couples with the outlets 140 on the inlet panel 132, and further includes a flue 169. The flue 169 is coupled to an outlet of the draft-inducing exhaust fan 162. Burned gases (i.e., products of combustion) may be delivered into flue 169 that vents the gases. In an assembled position, the draft-inducer assembly 160 is positioned relative to the heat exchanger assembly 130 such that manifold 164 at least substantially aligns with the outlets 140 and the flue 169 aligns with an opening in housing 108 to exhaust burned gases.

To move the air that is to be heated for the controlled space, a blower assembly 168 is suspended from the shelf 120. The blower assembly 168 includes a blower 172 with the blower outlet 176 that aligns with the opening 124 in the mounting shelf 120. When assembled and operating, air is pulled into sides of the blower 172 and then forced air exits the blower outlet 176 and goes through the opening 124 in the shelf 120 before going as an airstream over an exterior of the plurality of clamshell heat exchangers 104 before exiting the furnace 100 through a heated-air outlet of the housing 108.

A controller 183 is shown associated with the blower 172. The controller 183 may be operable to turn on the draft-inducing fan 162 to initiate a draft in the heat exchangers 104 in the heating zone 128 and purge potentially harmful unburned gases or gaseous combustion products. The controller 183 activates an igniter to warm up over a set period of time. Then the controller 183 opens the valve 148 to admit gas into the manifold 152 and the one or more gas orifices. The gas exits the orifice(s) and begins to mix with primary air to form a rich gas-air mixture inside the burner(s). Assuming ignition by the igniter (protocols are provided in case that is not the situation), the controller 183 then activates the blower 172, which forces the airstream upward through the opening 124 and across the heat exchangers 104. As the airstream passes over the surfaces of the heat exchangers 104, the air is warmed, whereupon it may be delivered or distributed as needed to provide heating in the space to be conditioned.

Referring now primarily to FIGS. 2-18, and initially to FIG. 2, an illustrative embodiment of a clamshell heat exchanger 104 is presented. The clamshell heat exchanger 104 has a first clamshell half 184 and a second clamshell half 188. The clamshell halves 184, 188 may be formed by shaping a sheet metal blank to form the two halves. The halves may be placed with the concave portions facing each other and joined by edge crimping or clamping along a peripheral edge 192 or fastening or welding. This may be done by having a lip portion 194 of one of the clamshell halves, e.g., clamshell half 188, extend further and be bent and crimped on the other clamshell half 184.

The joining of the two clamshell halves 184, 188 forms a clamshell body 196. The claimshell body 196 is shown with contour lines 197 for illustration purposes. The clamshell heat exchanger 104 includes an inlet 200 and an outlet 204. The inlet 200 receives combustion products from a burner assembly, e.g., burner assembly 144 (FIG. 1). The inlet 200 has an inlet collar 208 that slides through the vest panel, e.g., 132 in FIG. 1, and is rolled over to form an attachment. Similarly, the outlet 204 is formed with an outlet collar 212 that also slides through the vest panel and is rolled over for attachment.

The clamshell heat exchanger 104 in the assembled position includes a first passageway 216 having a first end 220 and a second end 224. The first passageway 216 is a combustion passageway that receives combustion products from the burner assembly as introduced through the inlet 200. The second end 224 of the first passageway is fluidly coupled with another portion of the clamshell heat exchanger 104, namely, a turnaround passageway 228.

The turnaround passageway 228 has a first end 232 and a second end 236. The first end 232 is coupled to the second end 224 of the first passageway 216. The turnaround passageway 228 is configured to change the direction of fluid flow received from the first passageway 216 by at least about 120 degrees and in some embodiments by 150 degrees or 180 degrees as shown. The second end 236 of the turnaround passageway 228 is coupled to a second passageway 240.

The second passageway 240 has a first end 244 and a second end 248. The first end 244 of the second passageway 240 is coupled to the second end 236 of the turnaround passageway 228. The outlet 204 is coupled to the second passageway 240 proximate the second end 248 of the second passageway 240. The second passageway 240 may include a first dimple 252 and a second dimple 256 that create a restriction and direct the gas flow around the dimples 252, 256 to cover a broader area of the second passageway 240. The second passageway 240 creates a plurality of flow streams, e.g., flow streams 260 and 264 in portions. A half dimple 268 may be formed on an inboard edge 272 of the second passageway 240. A plurality of veins 282 in the second passageway 240 create a number of flow streams 260, 264, e.g., two or three or more. The veins 282 direct flue gas flow to maximize heat transfer and efficiency while minimizing flow restriction (ie pressure drop) through passageway 240.

The first passageway 216 is fluidly coupled to the inlet 200 on the first end 220 and to the turnaround passageway 228 at the second end 224. The first end 244 of the second passageway 240 is fluidly coupled to the second end 236 of the turnaround passageway 228 and to the outlet 204 at the second end 248. Together, the first passageway 216, turnaround passageway 228, and second passageway 240 form a flow pathway for combustion products through the clamshell heat exchanger 104. The combustion products enter the inlet 200, travel through the first passageway 216, turn at the turnaround passageway 228, go through the second passageway 240, and exit at the outlet 204 from where the flow may go to a draft inducer. Meanwhile, a blower assembly, e.g., blower assembly 168, moves an airstream 274 across an exterior of the clamshell heat exchanger 104 to transfer heat thereto. The heated airstream 274 may then be delivered to a space that is to be conditioned or controlled.

The first passageway 216 has a lateral cross section that has more area above a lateral center line 276, than below the lateral center line. This is seen clearly in FIGS. 3A, 3B, and 18 a cross section of the first passageway 216 has more area in an upper portion than a lower portion. With reference to FIG. 18, a lateral centerline (horizontal midline) 276 (directions for orientation shown) defines a first area 280 above the lateral centerline 276 and a second area 284 below the lateral centerline 276. Note also while the first passageway 216 is symmetric about a vertical midline 277, the first passageway is not symmetric about the lateral centerline 276. The overall shape of a lateral cross section or combined cross sectional area 288 is that of an upside down tear drop or an airfoil with the leading edge on top 278 for the orientation shown, which is also saying the wider portion is closest to the second passageway 240 as is clear in FIG. 3A. The shape may help to reduce fatigue stress and increase the life of the heat exchanger. FIG. 19 shows a portion of another embodiment of a clamshell heat exchanger 104 with a cross section of the first passageway 216 that has more cross section area above the lateral centerline 276 than below the lateral centerline 276.

The clamshell heat exchanger 104 includes one or more seal portions or flanges 292 with one or more eyelets 296. The two clamshell halves 184, 188 are attached to each other to avoid leaks from within the passageways 216, 228, 240.

In the illustrative embodiment of FIG. 2, there are only two passes of the heated gases and not three or some other number. The passes are straight.

Referring now primarily to FIGS. 4-15, details of one illustrative embodiment are presented. It will be appreciated by one skilled in the art that the dimensions may vary in different applications and with different cabinet sizes. The illustrative dimensions for FIG. 4 are in Table I below.

TABLE I Numeral Description or Name Dimension (inches) 300 Length (Longitudinal) 19.97 304 Length 1.12 308 Length 13.39 312 Length 5.47 316 Length 0.46 320 Length 1.12 324 Length 5.06 328 Length 9.00 332 Length 12.93 336 Length 14.51 340 Length 16.87 344 Length 18.22 348 Length of angled portion 1.42 352 Length of angled portion 3.50 356 Outer Diameter 0.559 360 Outer Diameter 1.10 364 Outer Diameter 1.10 368 Height 8.13 372 Height 3.02 376 Height 4.10 380 Height 3.5 384 Height 3.56 388 Height 4.01 392 Height 3.47 396 Height 3.19 400 Height 2.84 404 Height 4.62 408 Height 1.25 412 Height 2.29

It will be apparent that the first passageway 216 diverges in the direction of the fluid flow along the flow pathway. For example, at approximately 25% of the distance from just beyond the inlet 200 to the far end of the turnaround passageway 228, the height 400 is 2.84 inches and at approximately 50% the height 396 is 3.19 inches, which is a 12% greater in height. At approximately the 75% point, the height 384 is 3.56 inches, which is about 25% greater than the height at the 25% point. The first passageway 216 diverges along a majority of the flow pathway from the inlet 200 to the turnaround passageway 228. In one illustrative embodiment, the first passageway 216 has a first end 220 proximate the inlet 200 and a second end 224 opposite the first end 220, and the first passageway 216 diverges from proximate the first end 220 to the second end 224 by at least 35%.

With reference now primarily to FIG. 16, a slope of the diverging walls in the first passageway 216 may be determined. In this embodiment, with respect to the inner wall 416 (closest to the second passageway 240) and outer wall 420 (closest to the peripheral edge 192) of the first passageway 216, the outer wall 420 is substantially straight in elevation view and the other wall, inner wall 416, is angled. The inner wall 416 expands with a slope between the outer wall 420 and the inner wall 416 of the first passageway 216 of approximately 0.08 for at least a majority of a longitudinal length of the first passageway 216. For example, the length between a 25% point and a 50% point of the horizontal (for orientation shown) length of the first passageway 216 is 3.94 inches (see numeral 424) and the rise is 0.35 inches (see numeral 428), and accordingly, the slope is 0.35/3.94, which is 0.088. The slope of the first passageway 216 generally will have a range of about 0.07 to 0.09.

Referring again primarily to FIG. 4, the second passageway 240 (I don't see 240 shown in FIG. 4) also diverges in the direction of flow along the flow pathway for most of its longitudinal length. So, with reference from a distance just beyond the outlet 204 to the back of the turnaround passageway 228, the dimensions are as follows: at approximately the 75% point the height 388 is 4.01; and at approximately the 25% point the height 404 is 4.62. So that is an increase of about 15%; in some embodiments the range is 13% to 18%. In one illustrative embodiment, the second passageway 240 diverges from proximate the first end 244 of the second passageway 240 to the widest portion of the second passageway 240 by at least 14%. A slope for the diverging walls in the second passageway 240 can also be calculated.

With reference now primarily to FIG. 17, the horizontal difference between the locations where heights 388 and 404 are taken is 7.87 inches (see numeral 426) and the difference in the heights are 0.61 inches (see numeral 430). So, the slope is 0.61/7.87 which is 0.077. In other embodiments, the range may be 0.07 to 0.12.

The final portion 432 of the second passageway 240 that is about 10% to 18% of the longitudinal length of the second passageway 240 has a steep slope back to the outlet 204; in some embodiments, the slope is in the range of −1.25 to −2.5, and in the embodiment shown is approximately −2.3. The negative sign is for the orientation shown in FIG. 17 but may be left off in some representations here, but the reader will understand the direction.

In some embodiments, the first passageway 216 diverges along the flow path only. In others, both the first passageway 216 and the second passageway 240 diverge along a majority of the flow path for each. The first passageway 216 diverges along a majority of the flow pathway from the first end 220 to the second end 224 of the first passageway 216 and the second passageway 240 diverges along a portion of the flow pathway from the first end 244 to the second end 248 of the second passageway 240. In one embodiment, the two passages 216, 240 both diverge with a slope of 0.08 (orientation of FIGS. 16 and 17, respectively) over a majority of their respect longitudinal lengths.

Referring now primarily to FIGS. 5-15, an illustrative embodiment of the clamshell heat exchanger 104 is shown with lateral cross sections (FIGS. 6-15) taken at the various locations shown in FIG. 5 going from the left to the right in that figure. FIG. 6 is near a far end of the turnaround passageway 228. FIG. 7 is the interface of the first passageway 216, the second passageway 240 and the turnaround passageway 228. FIG. 8 shows a portion just to the right of FIG. 7 in FIG. 5 and shows a baffle 436 that creates two channels having two flow streams. The flow steams are accentuated further downstream as shown in FIG. 9 by the dimple 252. The two flow streams help keep the flue-side pressure losses within a desired range while maintaining heat transfer at a desired level. FIG. 10 is taken at the half dimple 268. FIG. 11 is at the next dimple 256. FIGS. 12-15 are at the final downstream portion of the flow pathway.

Referring now primarily to FIG. 20, another illustrative embodiment of a clamshell heat exchanger 104 is shown. This embodiment includes three dimples 252, 256, and 440 in the second passageway 240 to create additional flow streams 264. The embodiment shown has three flow streams.

In some embodiments, the clamshell heat exchangers 104 shown above may include additional features. For example, a folding pattern of the lip 194 over the other clamshell half may be used. “W” shaped stamping may be the pattern between the first passageway and the second passageway to prevent short circuiting of the combustion gases and resultant loss in efficiency. Of course, perimeter profiles of the heat exchanger may be modified to fit into particular furnace applications.

Although the present invention and its advantages have been disclosed in the context of certain illustrative, non-limiting embodiments, it should be understood that various changes, substitutions, permutations, and alterations can be made without departing from the scope of the invention as defined by the claims. It will be appreciated that any feature that is described in a connection to any one embodiment may also be applicable to any other embodiment. 

What is claimed:
 1. A clamshell heat exchanger for use in a direct combustion furnace of an HVAC system, the clamshell heat exchanger comprising: a first clamshell half; a second clamshell half; wherein coupling the first clamshell half to the second clamshell half forms an assembled clamshell heat exchanger comprising: an inlet and an outlet, a first passageway, which is a combustion passageway, having a first end and a second end, wherein the inlet is proximate the first end, a turnaround passageway having a first end and second end, wherein the second end of the first passageway is fluidly coupled to the first end of the turnaround passageway, a second passageway, which is an exhaust passageway, having a first end and a second end, wherein the first end is fluidly coupled to the second end of the turnaround passageway and wherein the outlet is proximate the second end of the second passageway, wherein the first passageway is fluidly coupled to the turnaround passageway and the second passageway is fluidly coupled to the turnaround passageway to form a flow pathway for combustion products, and wherein a lateral cross section of the first passageway has more area above a lateral centerline than below the lateral centerline.
 2. The clamshell heat exchanger of claim 1, wherein the lateral cross section of the first passageway is shaped like an upside down tear drop.
 3. The clamshell heat exchanger of claim 1, wherein the lateral cross section of the first passageway is shaped like an airfoil with a leading edge facing upward.
 4. The clamshell heat exchanger of claim 1, wherein the first passageway diverges along a majority of the flow pathway from the inlet to the turnaround passageway.
 5. The clamshell heat exchanger of claim 1, wherein the first passageway diverges along a majority of the flow pathway from the first end to the second end of the first passageway and wherein the second passageway diverges along a portion of the flow pathway from the first end to the second end of the second passageway.
 6. The clamshell heat exchanger of claim 1, wherein the first passageway diverges along a majority of a flow pathway from the first end to the second end with a slope of at least 0.08; and wherein the second passageway diverges along a majority of a flow pathway from the first end to the second end with a slope of at least 0.08.
 7. The clamshell heat exchanger of claim 1, further comprising a baffle formed on an inboard portion of the second passageway forming two flow streams.
 8. The clamshell heat exchanger of claim 1, further comprising a first dimple and a second dimple on an inboard portion of the second passageway.
 9. The clamshell heat exchanger of claim 1, wherein the flow pathway increases in height along a majority of a length of the flow pathway.
 10. The clamshell heat exchanger of claim 1, wherein the second passageway has a outer wall and an inner wall that converge at least within a final ten percent of the flow pathway proximate the outlet with a slope of at least 1.25 towards the outlet.
 11. The clamshell heat exchanger of claim 1, wherein the lateral cross section of the first passageway is shaped like an upside down teardrop; wherein the first passageway diverges along a flow pathway from the first end to the second end with a slope of at least 0.08 for a majority of the flow pathway in the first passageway; wherein the second passageway diverges along the flow pathway from the first end to the second end with a slope of at least 0.08 for a majority of the flow pathway in the second passageway; further comprising a baffle formed on an inboard portion of the second passageway forming two flow streams; and wherein the second passageway has a top wall and a bottom wall that converge within a final ten percent of the flow pathway proximate the outlet with a slope of at least 1.25.
 12. A clamshell heat exchanger for use in a combustion furnace, the clamshell heat exchanger comprising: a first clamshell half; a second clamshell half; wherein, when coupled, the first clamshell half and the second clamshell half form the clamshell heat exchanger; a first passageway coupled to an inlet for receiving combustion products from a burner assembly, the first passageway having a longitudinal length in a first direction and comprising a first portion of a flow pathway; a turnaround passageway coupled to the first passageway and comprising a second portion of a flow pathway that receives fluids going in a first direction and turns the fluids at least 120 degrees; a second passageway coupled to the turnround passageway and having a longitudinal length in the first direction and comprising a third flow pathway that is opposite in direction to the first flow pathway and that comprises two flow streams; and wherein the first passageway has a lateral cross section resembling an airfoil with its leading edge closest to the second passageway.
 13. The clamshell heat exchanger of claim 12, wherein the lateral cross section of the first passageway has a first area to one side of a lateral centerline, wherein the first area is closest to the second passageway with respect to the lateral centerline, and wherein the first area is greater than a second area that is on an opposite side of the lateral centerline.
 14. The clamshell heat exchanger of claim 12, further comprising a baffle formed on an inboard portion of the second passageway forming two flow streams.
 15. A clamshell heat exchanger for a combustion furnace, the clamshell heat exchanger comprising: a heat exchanger body formed with an inlet and an outlet; a first passageway, turnaround passageway, and a second passageway formed in the heat exchanger body that together define a flow pathway from the inlet through the first passageway, then the turnaround passageway, then the second passageway, and finally to the outlet; wherein an outer wall portion and an inner wall portion of the first passageway diverge from one another; and wherein the first passageway is not symmetric about a lateral centerline.
 16. The clamshell heat exchanger of claim 15, wherein a lateral cross section of the first passageway is shaped like a teardrop having a widest portion closest to the second passageway.
 17. The clamshell heat exchanger of claim 15, with respect to the inner and outer wall of the first passageway, one wall is substantially straight in elevation view and the other is angled, and wherein a slope between the outer wall and the inner wall of the first passageway is greater than or equal to 0.08 for at least a majority of a longitudinal length of the first passageway.
 18. The clamshell heat exchanger of claim 15, wherein for at least a majority of a longitudinal length of the second passageway, an outer wall and an inner wall of the second passageway diverge from one another.
 19. The clamshell heat exchanger of claim 15, wherein the clamshell body comprises a first clamshell half and a second clamshell half coupled at least along a peripheral edge.
 20. The clamshell heat exchanger of claim 15, further comprising a baffle formed on the clamshell body in the second passageway to form two flow streams.
 21. The clamshell heat exchanger of claim 15, wherein the first passageway has a first end proximate the inlet and a second end opposite the first end, and the first passageway diverges from proximate the first end to the second end by at least 35%.
 22. The clamshell heat exchanger of claim 15, wherein the second passageway has a first end proximate the turnaround passageway and a second end proximate the outlet, and the second passageway diverges from proximate the first end of the second passageway to a widest portion of the second passageway by at least 14%. 